Over 2 million people in the United States have experienced unprovoked seizures or been diagnosed with epilepsy. In approximately 25% of cases, seizures are refractory to medical therapies. Inability to effectively treat epilepsy reflecs a lack of understanding of the basic mechanisms of this disorder. Up to 50% of traumatic brain injury survivors develop epilepsy. Posttraumatic epilepsy (PTE) is associated with alterations in hippocampal circuits including cell loss and reactive plasticity. In the mammalian brain, there is continual generation of new neurons in a few key brain regions throughout adulthood. This process, referred to as adult neurogenesis, represents a form of experience-dependent plasticity that is believed to support normal brain function. Brain insults including traumatic bran injury, seizures, and stroke are associated with increases in hippocampal adult neurogenesis, and abnormal integration of adult-born neurons within hippocampal circuitry may provide a substrate for hyperexcitable circuits that contribute to seizures. Epilepsy is associated with the emergence of adult-born dentate granule cells (DGCs) that display abnormal dendritic fields and axons that may project to unexpected targets. Adult-born DGCs are associated with spontaneous seizures in experimental epilepsy and blockade of adult neurogenesis reduces spontaneous seizure expression in an animal model of PTE. Despite these reports, inherent limitations of the techniques used have prevented the characterization of functional cellular connections formed by adult-born neurons with their synaptic targets. This proposal aims to develop a new technique to selectively label and stimulate newly-born neurons in the adult brain and then use this technique to assess functional outputs of hippocampal adult-born neurons in a mouse model of PTE. Work here aims to: 1) Selectively target expression of channelrhodopsin (ChR2) in adult-born progenitor cells based on their tamoxifen-inducible expression of nestin using Nestin-Cre mice. Nestin-Cre mice will be administered with a Cre-inducible adeno-associated virus (DIO-AAV) with double- floxed reverse cassettes containing channelrhodopsin (ChR2) and the fluorescent report mCherry (abbreviation: ChR2-mCherry; construct: pAAV-Ef1a-DIO-hChR2(H134R)-mCherry-WPRE-pA); and 2) Use blue-light stimulation parameters to activate adult-born neurons and drive signaling to their postsynaptic targets. Whole-cell patch-clamp recordings will be performed on DGCs in hippocampal slices from Nestin-Cre mice that have received injections of the ChR2-mCherry construct to describe the functional projections formed by adult-born neurons after brain injury. Improved understanding of how adult-born neurons incorporate into neural networks and signal during normal and PTE states will help define their relevance as therapeutic targets and will also provide new context for evaluation of clinically available drugs that have documented effects on adult-born cells.